Negative electrode active material, mixed negative electrode active material, and method for producing negative electrode active material

ABSTRACT

A negative electrode active material includes a negative electrode active material particle. The negative electrode active material particle includes a silicon compound particle including a silicon compound (SiO x : 0.5≤x≤1.6). The silicon compound particle includes crystalline Li 2 Si 2 O 5  in at least part of the silicon compound particle. Among peak intensities A derived from Li 2 SiO 3 , B derived from Si, a peak intensity C derived from Li 2 Si 2 O 5 , and D derived from SiO 2  which are obtained from a  29 Si-MAS-NMR spectrum of the silicon compound particle, the peak intensity C is the highest intensity, and the peak intensity A and the peak intensity C satisfy a relationship of the formula A&lt;C/3. Thus, a negative electrode active material stabilizes a slurry prepared when a negative electrode of a secondary battery is prepared, and improves initial charge and discharge and cycle characteristics when used as a negative electrode active material for a secondary battery.

TECHNICAL FIELD

The present invention relates to a negative electrode active material, amixed negative electrode active material, and a method for producing anegative electrode active material.

BACKGROUND ART

In recent years, small electronic devices represented by a mobileterminal and so forth is widely used, whereby further downsizing, weightreduction, and prolonged life are strongly requested. To meet suchmarket needs, a secondary battery which can especially achievedownsizing, weight reduction, and high energy density is beingdeveloped. This secondary battery is being investigated for applicationnot only to small electronic devices but also to large electronicdevices represented by automobiles and the like as well as an electricpower storage system represented by houses.

Among them, a lithium ion secondary battery has high expectation becausedownsizing and high capacity can be easily achieved and also a highenergy-density can be obtained as compared with a lead battery or anickel cadmium battery.

The lithium ion secondary battery as mentioned above includes a positiveelectrode, a negative electrode, a separator, and an electrolyticsolution. The negative electrode includes a negative electrode activematerial which involves in the charge and discharge reactions.

In the negative electrode active material, a carbon material is widelyused, while a further increase in the battery capacity is requested fromrecent market needs. In order to increase the battery capacity, as thenegative electrode active material use of silicon is being studied. Thisis because the theoretical capacity of silicon (4199 mAh/g) is 10 timesor more as much as the theoretical capacity of graphite (372 mAh/g), sothat a significant increase in the battery capacity can be expected.Research and development of a silicon material as the material for thenegative electrode active material are carried out with regard not onlyto silicon simple substance but also to compounds represented by analloy and an oxide thereof. Besides, with regard to the form of theactive material, the study has been made from a coating type which isstandard in the carbon material to an integral type directly depositedonto a current collector.

However, when silicon is used as a main raw material in the negativeelectrode active material, the negative electrode active materialundergoes expansion and contraction during charge and discharge, so thata crack can readily occur mainly near the surface of the negativeelectrode active material. In addition, inside the active material anionic substance is formed and may cause a crack in the negativeelectrode active material. When the surface of the negative electrodeactive material is cracked, a new surface is formed so that the reactivearea of the active material increases. In this event, a decompositionreaction of the electrolytic solution takes place on the new surface,and also the electrolytic solution is consumed because a film of thedecomposition product of the electrolytic solution is formed on the newsurface. Accordingly, the cycle characteristics easily deteriorate.

Until today, in order to improve the battery's initial efficiency andcycle characteristics, various investigations have been made with regardto the electrode configuration as well as the negative electrodematerial for the lithium ion secondary battery mainly composed of asilicon material.

Specifically, in order to obtain excellent cycle characteristics andhigh safety, silicon and amorphous silicon dioxide are simultaneouslydeposited by using a gas phase method (for example, see Patent Document1). In order to obtain high battery capacity and high safety, a carbonmaterial (electron conductive material) is disposed on the surface ofsilicon oxide particles (for example, see Patent Document 2). In orderto improve the cycle characteristics and to obtain the high input/outputcharacteristics, an active material containing silicon and oxygen isproduced, and also an active material layer having a high oxygen rationear a current collector is formed (for example, see Patent Document 3).In order to improve the cycle characteristics, a silicon active materialis made to contain oxygen with an average oxygen content of 40 at % orless and also to have a higher oxygen content near a current collector(for example, see Patent Document 4).

Further, in order to improve the first time charge and dischargeefficiency, a nano composite including an Si phase, SiO₂, and M_(y)Ometal oxide is used (for example, see Patent Document 5). In order toimprove the cycle characteristics, a mixture of SiO_(x) (0.8≤x≤1.5 and aparticle diameter range of 1 to 50 μm) with a carbon material is burnedat high temperature (for example, see Patent Document 6). In order toimprove the cycle characteristics, a molar ratio of oxygen to silicon ina negative electrode active material is made 0.1 to 1.2, and the activematerial is controlled such that the difference between the maximum andminimum values of the molar ratio near the interface of the activematerial and the current collector is in the range of 0.4 or less (forexample, see Patent Document 7). In order to improve the battery loadcharacteristics, a metal oxide including lithium is used (for example,see Patent Document 8). In order to improve the cycle characteristics, ahydrophobic layer of a silane compound or the like is formed on thesilicon material surface layer (for example, see Patent Document 9). Inorder to improve the cycle characteristics, silicon oxide is used, andon the surface layer thereof, a graphite coating film is formed so as toprovide conductivity (for example, see Patent Document 10). In PatentDocument 10, with regard to the shift values obtained from the Ramanspectrum of the graphite film, broad peaks appear at 1330 cm⁻¹ and 1580cm⁻¹ with the intensity ratio I₁₃₃₀/I₁₅₈₀ being 1.5<I₁₃₃₀/I₁₅₈₀<3. Inorder to obtain a high battery capacity and to improve the cyclecharacteristics, particles having a silicon microcrystal phase dispersedin silicon dioxide are used (for example, see Patent Document 11). Inorder to improve the over-charge and over-discharge characteristics, asilicon oxide is used in which the atom number ratio of silicon tooxygen is controlled at 1:y (0<y<2) (for example, see Patent Document12).

CITATION LIST Patent Literature

Patent Literature 1: JP 2001-185127 A

Patent Literature 2: JP 2002-042806 A

Patent Literature 3: JP 2006-164954 A

Patent Literature 4: JP 2006-114454 A

Patent Literature 5: JP 2009-070825 A

Patent Literature 6: JP 2008-282819 A

Patent Literature 7: JP 2008-251369 A

Patent Literature 8: JP 2008-177346 A

Patent Literature 9: JP 2007-234255 A

Patent Literature 10: JP 2009-212074 A

Patent Literature 11: JP 2009-205950 A

Patent Literature 12: JP 2997741 B1

SUMMARY OF INVENTION Technical Problem

As mentioned above, in recent years, in small electronic devicesrepresented by a mobile terminal and so forth, enhancement ofperformance and multi-functionalization are being progressing, therebyrequiring an increase in the battery capacity of a lithium ion secondarybattery, which is a main power source thereof. As one method to solvethis problem, the lithium ion secondary battery provided with a negativeelectrode using a silicon-based active material as a main materialthereof is desired to be developed. In addition, the lithium ionsecondary battery using the silicon-based active material is requestedto be almost equivalent to the lithium ion secondary battery using acarbon-based active material in terms of a first time efficiency, cyclecharacteristics, and a stability of an aqueous slurry prepared when thenegative electrode is prepared. However, a negative electrode activematerial showing the first time efficiency, cycle stability, and slurrystability that are equivalent to those of the lithium ion secondarybattery using the carbon-based active material has not been proposedyet.

The present invention was made in view of the problems mentioned above,and it is an object of the present invention to provide: a negativeelectrode active material that can stabilize a slurry prepared when anegative electrode of a secondary battery is prepared and that canimprove the cycle characteristics as well as the initial charge anddischarge characteristics when this negative electrode active materialis used as a negative electrode active material for the secondarybattery; and a mixed negative electrode active material including thenegative electrode active material. Also, it is an object of the presentinvention to provide a method for producing the negative electrodeactive material that can stabilize a slurry prepared when the negativeelectrode is prepared and that can improve the initial charge anddischarge characteristics and cycle characteristics.

In addition, another object of the present invention is to provide amethod for producing a negative electrode active material excellent inbattery characteristics and slurry stability as described above.

Solution to Problem

In order to achieve the objects mentioned above, the present inventionprovides a negative electrode active material comprising a negativeelectrode active material particle, wherein

the negative electrode active material particle comprises a siliconcompound particle comprising a silicon compound (SiO_(x): 0.5≤x≤1.6),

the silicon compound particle comprises crystalline Li₂Si₂O₅ in at leastpart of the silicon compound particle,

among a peak intensity A derived from Li₂SiO₃, a peak intensity Bderived from Si, a peak intensity C derived from Li₂Si₂O₅, and a peakintensity B derived from SiO₂ which are obtained from a ²⁹Si-MAS-NMR(²⁹Si-Magic Angle Spinning-Nuclear Magnetic Resonance) spectrum of thesilicon compound particle, the peak intensity C is the highestintensity, and

the peak intensity A and the peak intensity C satisfy a relationship ofthe following formula 1,A<C/3.  formula 1

Because the negative electrode active material of the present inventionincludes the negative electrode active material particle including thesilicon compound particle (also referred to as silicon-based activematerial particle), the battery capacity can be improved. In addition,because the silicon compound particle includes lithium silicate asdescribed above, an irreversible capacity generated during charging canbe reduced. Moreover, Li₂Si₂O₅ has higher water resistance than Li₄SiO₄;further, Li₂Si₂O₅ has higher water resistance than Li₂SiO₃, which hasrelatively high water resistance among Li silicates. Thus, in a casewhere the peak intensity C is the highest and the condition of theformula 1 is satisfied, the water resistance is further improved and theslurry stability is further increased.

At this time, the peak intensity A and the peak intensity C in the²⁹Si-MAS-NMR spectrum preferably further satisfy a relationship of thefollowing formula 2,A≤C/5.  formula 2

In this manner, when the intensity C thus obtained is much higher thanthe peak intensity A, the water resistance is especially improved andthe slurry stability is especially increased.

In addition, a peak preferably appears in a region near a chemical shiftvalue of −130 ppm in the ²⁹Si-MAS-NMR spectrum.

The peak appearing near the chemical shift value of −130 ppm ispresumably derived from amorphous silicon. Hence, the ratio of theamorphous silicon component is higher, making it possible to furtherimprove the cycle characteristics.

In addition, in the silicon compound particle, preferably, a half-valuewidth (2θ) of a diffraction peak attributable to a Si (111) crystalplane obtained by an X-ray diffraction is 1.2° or more, and acrystallite's size corresponding to the crystal plane is 7.5 nm or less.

When the negative electrode active material including the siliconcompound particle having the above silicon crystallinity is used as anegative electrode active material for a lithium ion secondary battery,more excellent cycle characteristics and initial charge and dischargecharacteristics are obtained.

In addition, preferably, a test cell is prepared, the test cellcomprising a counter lithium and a negative electrode, which comprises amixture of the negative electrode active material and a carbon-basedactive material;

in the test cell, charge in which a current is flowed so as to insertlithium into the negative electrode active material and discharge inwhich a current is flowed so as to release lithium from the negativeelectrode active material are executed for 30 times; and

when a graph is drawn showing a relationship between an electricpotential V of the negative electrode and a differential value dQ/dV,which is obtained by differentiating a discharge capacity Q in eachcharge and discharge with the electric potential V with a standard ofthe counter lithium, the differential value dQ/dV upon discharging onand after Xth time (1≤X≤30) has a peak in a range of 0.40 V to 0.55 V ofthe electric potential V of the negative electrode.

The peak in the V-dQ/dV curve mentioned above resembles the peak of asilicon material. Because the discharge curve in a higher electricpotential side rises sharply, a capacity can be readily expressed uponbattery design. In addition, when the peak appears within 30 times ofthe charge and discharge, a stable bulk is formed in the negativeelectrode active material.

In addition, the negative electrode active material particles preferablyhave a median diameter of 1.0 μm or more and 15 μm or less.

When the median diameter is 1.0 μm or more, increase in the irreversiblecapacity of the battery due to the increase in the surface area per masscan be suppressed. On the other hand, when the median diameter is 15 μmor less, the particles are difficult to be cracked, so that a newsurface is difficult to be emerged.

In addition, the negative electrode active material particle preferablycontains a carbon material at a surface layer portion.

In this manner, when the negative electrode active material particlecontains a carbon material at the surface layer portion, theconductivity can be improved.

In addition, the carbon material preferably has an average thickness of5 nm or more and 5000 nm or less.

When the carbon material has an average thickness of 5 nm or more, theconductivity can be improved. Meanwhile, when the carbon material to becoated has an average thickness of 5000 nm or less, the use of thenegative electrode active material including the negative electrodeactive material particle like this in a lithium ion secondary batterycan ensure sufficient amount of the silicon compound particle, so thatthe decrease in the battery capacity can be suppressed.

Moreover, the present invention provides a mixed negative electrodeactive material comprising:

the negative electrode active material; and

carbon-based active material.

In this manner, when the material for forming a negative electrodeactive material layer is made so as to include a carbon-based activematerial as well as the negative electrode active material of thepresent invention (silicon-based negative electrode active material),the conductivity of the negative electrode active material layer can beimproved, and also an expansion stress due to charging can be relaxed.In addition, mixing the silicon-based negative electrode active materialwith the carbon-based active material can increase the battery capacity.

At this time, a ratio of the negative electrode active material relativeto a total amount of the mixed negative electrode active material ispreferably 10% by mass or more.

When the ratio of the mass of the negative electrode active material(silicon-based negative electrode active material) relative to a totalmass of the negative electrode active material (silicon-based negativeelectrode active material) and the carbon-based active material is 10%by mass or more, the battery capacity can be further improved.

Further, to achieve the above object, the present invention provides amethod for producing a negative electrode active material whichcomprises a negative electrode active material particle comprising asilicon compound particle, the method comprising the steps of:

making silicon compound particles comprising a silicon compound(SiO_(x): 0.5≤x≤1.6);

inserting lithium into the silicon compound particles so as to includecrystalline Li₂Si₂O₅ in at least part of the silicon compound particlesto prepare negative electrode active material particles;

selecting, from the negative electrode active material particles, such anegative electrode active material particle that, among a peak intensityA derived from Li₂SiO₃, a peak intensity B derived from Si, a peakintensity C derived from Li₂Si₂O₅, and a peak intensity D derived fromSiO₂ which are obtained from a ²⁹Si-MAS-NMR spectrum of the negativeelectrode active material particle, the peak intensity C is the highestintensity, and the peak intensity A and the peak intensity C satisfy arelationship of the following formula 1; and

producing a negative electrode active material by using the selectednegative electrode active material particle,A<C/3.  formula 1

When the negative electrode active material is produced by selecting thenegative electrode active material particle as described above, anaqueous slurry prepared in preparing the negative electrode can bestabilized; in addition, the produced negative electrode active materialcan have a high capacity and also excellent cycle characteristics andinitial charge and discharge characteristics when used as a negativeelectrode active material for a lithium ion secondary battery.

Advantageous Effects of Invention

The negative electrode active material of the present invention iscapable of stabilizing an aqueous slurry prepared when the negativeelectrode is prepared. In addition, when the negative electrode activematerial of the present invention is used as a negative electrode activematerial for a secondary battery, a high capacity and excellent cyclecharacteristics and excellent initial charge and dischargecharacteristics are obtained. Moreover, the same effects are alsoobtained from the mixed negative electrode active material includingthis negative electrode active material. Further, the method forproducing a negative electrode active material of the present inventionmakes it possible to produce a negative electrode active material whichcan stabilize an aqueous slurry prepared in preparing the negativeelectrode, and which has excellent cycle characteristics and excellentinitial charge and discharge characteristics when used as a negativeelectrode active material for a lithium ion secondary battery.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing an example of a constitution ofa negative electrode for a non-aqueous electrolyte secondary battery,the negative electrode containing the negative electrode active materialof the present invention.

FIG. 2 is a figure illustrating a constitution example (laminate filmtype) of a lithium secondary battery containing the negative electrodeactive material of the present invention.

FIG. 3 is an example of a spectrum obtained when a silicon compoundparticle including amorphous silicon is measured by using ²⁹Si-MAS-NMR.

FIG. 4 is a ²⁹Si-MAS-NMR spectrum measured in Example 1-1.

FIG. 5 is X-ray diffraction spectra measured in Example 1-1 andComparative Example 1-2.

FIG. 6 is a ²⁹Si-MAS-NMR spectrum measured in Comparative Example 1-1.

FIG. 7 is a ²⁹Si-MAS-NMR spectrum measured in Comparative Example 1-2.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be explained;however, the present invention is not limited thereto.

As mentioned above, as one method to increase the battery capacity of alithium ion secondary battery, the use of a negative electrode using asilicon material as a main material has been studied as a negativeelectrode for the lithium ion secondary battery. It is desired that thelithium ion secondary battery using this silicon material have theslurry stability, initial charge and discharge characteristics, andcycle characteristics that are almost equivalent to those of the lithiumion secondary battery using a carbon-based active material. However, nonegative electrode active material having the slurry stability, initialcharge and discharge characteristics, and cycle characteristics that areequivalent to those of the lithium ion secondary battery using acarbon-based active material has been proposed yet.

Therefore, the present inventors have conducted earnest studies toobtain a negative electrode active material having a high batterycapacity and also excellent slurry stability, cycle characteristics, andfirst time efficiency when used for a secondary battery, therebybringing the present invention to the completion.

The negative electrode active material of the present invention includesa negative electrode active material particle. This negative electrodeactive material particle includes a silicon compound particle includinga silicon compound (SiO_(x): 0.5≤x≤1.6). The silicon compound particleincludes crystalline Li₂Si₂O₅ in at least part of the silicon compoundparticle. In addition, among a peak intensity A derived from Li₂SiO₃, apeak intensity B derived from Si, a peak intensity C derived fromLi₂Si₂O₅, and a peak intensity D derived from SiO₂ which are obtainedfrom a ²⁹Si-MAS-NMR (²⁹Si-Magic Angle Spinning-Nuclear MagneticResonance) spectrum of the silicon compound particle, the peak intensityC is the highest intensity, and the beak intensity A and the peakintensity C satisfy a relationship of the following formula 1,A<C/3.  formula 1

Since such a negative electrode active material includes thesilicon-based active material particle including the silicon compoundparticle, the battery capacity can be improved. In addition,incorporating lithium silicate as described above into the siliconcompound particle can reduce an irreversible capacity generated duringcharging. Moreover, since Li₂Si₂O₅ has higher water resistance and ismore stable than Li₄SiO₄ formed generally by employing anelectrochemical method or an oxidation-reduction method and Li₂SiO₃formed with a thermal energy of about 500° C., the slurry stability isimproved. Thus, in the case where the peak intensity C is the highestand the condition of the formula 1 is satisfied, that is, the peakintensity derived from Li₂Si₂O₅ is so high that the peak intensity Cthus obtained is sufficiently higher than A, the water resistance isfurther improved and the slurry stability is further increased.

Particularly preferably, the peak intensities A and C obtained from the²⁹Si-MAS-NMR spectrum satisfy the following formula 2,A≤C/5  formula 2

In this manner, when the content of Li₂Si₂O₅ is much higher thanLi₂SiO₃, the water resistance is especially improved.

Additionally, in the present invention, the peak intensities C and Bpreferably satisfy a relationship of C≥3B, and particularly preferablysatisfy a relationship of C≥5B. Similarly, the peak intensities C and Dpreferably satisfy a relationship of C≥3D, and particularly preferablysatisfy a relationship of C≥5D. In this manner, when the content ofLi₂Si₂O₅ is higher than those of Si and SiO₂, the cycle characteristicsare further improved. In addition, when C is higher than D, a largeportion of the SiO₂ component has been changed to Li₂Si₂O₅ in thisstate. Accordingly, in such a case, it is possible to sufficientlyreduce the irreversible capacity generated by the reaction between partof silicon dioxide and Li during charging and discharging of thebattery.

It should be noted however that, to determine how many times the peakintensity C should be higher than the other intensities so that theslurry can become stable, it is also necessary to consider that thedetermined peak intensity C somewhat varies depending on the compositionof the slurry to be prepared. According to the experimental resultsobtained by the present inventors so far, it can be said that if thepeak intensity C is 5 times or more as high as the other intensities,the slurry stability tends to improve more effectively in general.

In the ²⁹Si-MAS-NMR spectrum, the peak derived from Li₂SiO₃ appears neara chemical shift value of −75 ppm, the peak derived from Si appears neara chemical shift value of −86 ppm, the peak derived from Li₂Si₂O₅appears near a chemical shift value of −93 ppm, and the peak derivedfrom SiO₂ appears near a chemical shift value of −110 ppm.

The ²⁹Si-MAS-NMR spectrum measurement conditions may be ordinarymeasurement conditions. For example, the following conditions can beset:

²⁹Si MAS NMR

-   -   Instrument: 700 NMR spectrophotometer manufactured by Bruker        Corp.,    -   Probe: 4 mm HR-MAS rotor, 50 μL,    -   Sample rotation speed: 10 kHz,    -   Measured environment temperature: 25° C.

A peak intensity is represented by the height of the peak from a baseline calculated based on the ²⁹Si-MAS-NMR spectrum. In this event, thebase line can be determined by an ordinary method.

Moreover, in the ²⁹Si-MAS-NMR, spectrum, a peak preferably appears in aregion near a chemical shift value of −130 ppm. In such a case, theratio of the amorphous silicon component in the silicon compound ishigher, so that the cycle characteristics can be further improved. Notethat FIG. 3 shows an example of a peak derived from amorphous silicon.As in FIG. 3, the peak derived from amorphous silicon appears at aposition near a chemical shift value of −130 ppm in the ²⁹Si-MAS-NMRspectrum.

<Negative Electrode for Non-Aqueous Electrolyte Secondary Battery>

First, a negative electrode containing the negative electrode activematerial of the present invention will be explained. FIG. 1 is across-sectional view showing an example of a constitution of a negativeelectrode containing the negative electrode active material of thepresent invention.

[Constitution of Negative Electrode]

As illustrated in FIG. 1, a negative electrode 10 is configured to havea negative electrode active material layer 12 on a negative electrodecurrent collector 11. This negative electrode active material layer 12may be provided on both surfaces of the negative electrode currentcollector 11 or only on one surface thereof. Further, the negativeelectrode current collector 11 may not be used when the negativeelectrode active material of the present invention is used.

[Negative Electrode Current Collector]

The negative electrode current collector 11 is formed of an excellentconductive material which is also a mechanically strong material.Illustrative example of the conductive material usable in the negativeelectrode current collector 11 includes copper (Cu) and nickel (Ni). Itis preferable that the conductive material do not form an intermetalliccompound with lithium (Li).

It is preferable that the negative electrode current collector 11include carbon (C) and sulfur (S), in addition to main elements. This isbecause a physical strength of the negative electrode current collectorcan be increased by so doing, and because especially in the case thatthe negative electrode has an active material layer which expands uponcharging, if the current collector includes the elements mentionedabove, deformation of the electrode including the current collector canbe suppressed. Contents of these elements are not particularlyrestricted; but especially, it is preferable that content of eachelement be 70 ppm by mass or less. This is because a higher effect tosuppress the deformation can be obtained. Such an effect to suppressdeformation can further improve the cycle characteristics.

Surface of the negative electrode current collector 11 may be roughenedor not be roughened. Illustrative example of the roughened negativeelectrode current collector includes a metal foil having been subjectedto an electrolysis treatment, an emboss treatment, or a chemical etchingtreatment; and the like. Illustrative example of the negative electrodecurrent collector that is not roughened includes a rolled metal foil,and the like.

[Negative Electrode Active Material Layer]

The negative electrode active material 12 includes the negativeelectrode active material of the present invention which can adsorb andrelease a lithium ion; and in addition, in view of a battery design, itmay include other materials such as a negative electrode binder and aconductive assistant. The negative electrode active material includesthe negative electrode active material particle, and the negativeelectrode active material particle includes the silicon compoundparticle including the silicon compound (SiO_(x): 0.5≤x≤1.6).

In addition, the negative electrode active material layer 12 may includea mixed negative electrode active material including the negativeelectrode active material of the present invention (silicon-basednegative electrode active material) and a carbon-based active material.With these, electric resistance of the negative electrode activematerial layer can be lowered and also an expansion stress due tocharging can be relaxed. Illustrative example of the carbon-based activematerial usable includes thermally decomposed carbons, cokes, glass-likecarbon fibers, fired bodies of organic polymer compounds, carbon blacks,and the like.

In the mixed negative electrode active material, it is preferable thatthe ratio of the mass of the silicon-based negative electrode activematerial relative to the total mass of the silicon-based negativeelectrode active material and the carbon-based active material be 10% bymass or more. When the ratio of the silicon-based negative electrodeactive material is 10% by mass or more, the battery capacity can befurther improved.

The negative electrode active material of the present invention is asilicon oxide material which includes the silicon compound particle, andthe silicon compound particle includes the silicon compound (SiO_(x):0.5≤x≤1.6) as mentioned before; in the composition thereof, it is morepreferable when x approach near to 1. This is because an excellent cyclecharacteristics can be obtained by so doing. Meanwhile, the compositionof the silicon compound in the present invention does not necessarilymean a purity of 100%, so that it may contain minute amounts of impurityelements.

In the negative electrode active material of the present invention, thesilicon compound particle includes crystalline Li₂Si₂O₅ as describedabove. This is a material obtained by reforming a SiO₂ component portionto different lithium silicate in advance in the silicon compound; theSiO₂ component portion would be destabilized upon insertion and releaseof lithium during charging and discharging of the battery. Thisreformation enables a reduction in the irreversible capacity generatedupon charging.

The silicon compound particle preferably includes crystalline Li₂Si₂O₅,as the main Li silicate. In other words, the content of crystallineLi₂Si₂O₅ is preferably the highest among Li silicates. Moreover, inaddition to this Li silicate, Li₂SiO₃ and Li₄SiO₄ may be present in thebulk of the silicon compound particle. Thereby, the batterycharacteristics are further improved. Especially, Li₂SiO₃ is somewhatmore easily dissolved in water than Li₂Si₂O₅, but the silicon compoundparticle including Li₂SiO₃ can further improve the cyclecharacteristics. Meanwhile, these lithium silicates can be quantified byNMR as described above. Besides, quantification by XPS (X-rayphotoelectron spectroscopy) is possible. NMR measurement can be made bythe conditions as described above. XPS measurement can be made, forexample, by the following conditions.

XPS

-   -   Instrument: X-ray photoelectron spectroscopy instrument,    -   X-ray source: mono-colored Al Kα beam,    -   X-ray spot diameter: 100 μm,    -   Ar ion gun sputtering condition: 0.5 kV/2 mm×2 mm.

In addition, in the silicon compound particle, preferably, a half-valuewidth (2θ) of a diffraction peak attributable to a Si (111) crystalplane obtained by an X-ray diffraction using Cu-Kα ray is 1.2° or more,and a crystallite's size corresponding to the crystal plane is 7.5 nm orless. This peak appears near 2θ=28.4±0.5° when the crystallinity is high(when the half-value width is narrow). Silicon crystallinity of thesilicon compound in the silicon compound particle is better as thecrystallinity thereof is lower; and thus, especially when amount of theSi crystal is small, the battery characteristics can be improved andalso a stable Li compound can be produced.

Moreover, in the negative electrode active material of the presentinvention, the negative electrode active material particle preferablycontains a carbon material at a surface layer portion. When the negativeelectrode active material particle contains a carbon material at thesurface layer portion of the negative electrode active materialparticle, the conductivity can be improved. Accordingly, when thenegative electrode active material including such a negative electrodeactive material particle is used as a negative electrode active materialfor a secondary battery, the battery characteristics can be improved.

Further, the carbon material at the surface layer portion of thenegative electrode active material particle preferably has an averagethickness of 5 nm or more and 5000 nm or less. When the carbon materialhas an average thickness of 5 nm or more, the conductivity can beimproved. When the carbon material as the coating film has an averagethickness of 5000 nm or less, the decrease in the battery capacity canbe suppressed by using the negative electrode active material includingsuch a negative electrode active material particle as a negativeelectrode active material for a lithium ion secondary battery.

The average thickness of this carbon material can be calculated, forexample, by the procedure shown below. First, the negative electrodeactive material particle is observed under TEM (transmission electronmicroscope) with an arbitrary magnification. With regard to themagnification, it is preferable that with this magnification thethickness of the carbon material can be visually confirmed so as tomeasure the thickness. Next, the thickness of the carbon material ismeasured in arbitrary 15 spots. At this time, it is preferable to setthe measurement spots widely and randomly, not to concentrate to acertain spot as much as possible. Finally, the average value of thethicknesses of the 15 spots in the carbon material is calculated.

The covering rate of the carbon material is not particularly restricted;however, the rate as high as possible is preferable. The covering rateof 30% or more is preferable because the electric conductivity can beimproved furthermore. The covering method of the carbon material is notparticularly restricted; however, a sugar carbonization method and athermal decomposition method of a hydrocarbon gas are preferable. Thisis because these methods can improve the covering rate.

In addition, the negative electrode active material particles preferablyhave a median diameter (D₅₀: particle diameter when the cumulativevolume reaches 50%) of 1.0 μm or more and 15 μm or less. This is becausewhen the median diameter is within the above-mentioned range, storingand release of the lithium ion upon charging and discharging isfacilitated, and also the particles are difficult to be cracked. Whenthe median diameter is 1.0 μm or more, the surface area per a unit masscan be made small, so that increase in the irreversible capacity of thebattery can be suppressed. On the other hand, when the median diameteris 15 μm or less, the particles are difficult to be cracked, so that anew surface is difficult to be emerged.

The negative electrode active material (silicon-based active material)of the present invention is preferably such that a test cell is preparedwhich includes a counter lithium and a negative electrode including amixture of the silicon-based active material and a carbon-based activematerial; in the test cell, charge in which a current is flowed so as toinsert lithium into the silicon-based active material and discharge inwhich a current is flowed so as to release lithium from thesilicon-based active material are executed for 30 times; and when agraph is drawn showing a relationship between an electric potential V ofthe negative electrode and a differential value dQ/dV, which is obtainedby differentiating a discharge capacity Q in each charge and dischargewith the electric potential V with standard of the counter lithium, thedifferential value dQ/dV upon discharging on and after Xth time (1≤X≤30)has a peak in a range of 0.40 V to 0.55 V of the electric potential V ofthe negative electrode. The peak in the V-dQ/dV curve mentioned aboveresembles the peak of a silicon material. Because the discharge curve ahigher electric potential side rises sharply, a capacity can be readilyexpressed upon battery design. In addition, when the peak appears within30 times of the charge and discharge, it can be determined that a stablebulk is formed in the negative electrode active material.

As to the negative electrode binder included in the negative electrodeactive material layer, for example, any one or more of a polymermaterial, a synthetic rubber, etc. can be used. Illustrative example ofthe polymer material includes polyvinylidene fluoride, polyimide,polyamide aramid, polyacrylic acid, lithium polyacrylate, carboxymethylcellulose, and the like. Illustrative example of the synthetic rubberincludes a styrene butadiene-based rubber, a fluorinated rubber,ethylene propylene diene, and the like.

As to the negative electrode conductive assistant, for example, any oneor more of carbon materials such as a carbon black, an acetylene black,a graphite, a Ketjen black, a carbon nanotube, and a carbon nanofibercan be used.

The negative electrode active material layer is formed, for example, bya coating method. The coating method is the method in which the negativeelectrode active material particle, the binder, etc. as well as, ifnecessary, the conductive assistant and the carbon material are mixed,the resulting mixture is dispersed in an organic solvent, water or thelike, and then the resultant is applied.

[Method for Producing Negative Electrode]

The negative electrode can be produced, for example, by the procedureshown below. At first, the method for producing the negative electrodeactive material to be used for the negative electrode will be explained.Negative electrode active material particles are prepared by firstlymaking the silicon compound particles including the silicon compound(SiO_(x): 0.5≤x≤1.6), which are included in the negative electrodeactive material particles. Next, Li is inserted into the siliconcompound particles so as to include crystalline Li₂Si₂O₅ in at leastpart of the silicon compound particles. In this manner, the negativeelectrode active material particles are prepared. Then, from theprepared negative electrode active material particles, such a negativeelectrode active material particle is selected that, among a peakintensity A derived from Li₂SiO₃, a peak intensity B derived from Si, apeak intensity C derived from Li₂Si₂O₅, and a peak intensity D derivedfrom SiO₂ which are obtained from a ²⁹Si-MAS-NMR spectrum of thenegative electrode active material particle, the peak intensity C is thehighest intensity, and the peak intensity A and the peak intensity Csatisfy a relationship of the above-described formula 1. Subsequently,the selected negative electrode active material particle is used toproduce a negative electrode active material.

Moreover, the preparation of the negative electrode active materialparticles may further include a step of covering the silicon compoundparticles with a carbon material. The covering step with a carbonmaterial can be performed before the step of inserting Li. The negativeelectrode active material particles prepared by covering the surfaces ofthe silicon compound particles with the carbon material are excellent inthe conductivity.

More specifically, the negative electrode active material can beproduced as follows. First, a raw material capable of generating asilicon oxide gas is heated under reduced pressure in the presence of aninert gas in the temperature range of 900 to 1600° C., so that thesilicon oxide gas is generated. Considering the presence of oxygen onthe metal silicon powder surface and minute amount of oxygen in areaction furnace, the mixing molar ratio preferably satisfies therelationship of 0.8<metal silicon powder/silicon dioxide powder<1.3.

The generated silicon oxide gas is solidified on an adsorbing plate anddeposited thereon. Next, the deposited silicon oxide is taken out underthe state that the temperature inside the reaction furnace is dropped to100° C. or lower; and then, the deposited silicon oxide is crushed andpulverized by using a ball mill, a jet mill, or the like. In this way,the silicon compound particle can be prepared. Meanwhile, the Sicrystallite in the silicon compound particle can be controlled bychanging the vaporization temperature or a heat treatment after theformation thereof.

Here, a layer of the carbon material may be formed on the surface layerof the silicon compound particle. As to the formation method of thecarbon material layer, a thermal decomposition CVD method is preferable.A method for forming the carbon material layer by the thermaldecomposition CVD method will be explained.

First, the silicon compound particle is set in a furnace. Next, ahydrocarbon gas is introduced into the furnace, and then, thetemperature inside the furnace is raised. The decomposition temperatureis not particularly restricted; however, 1200° C. or lower ispreferable, more preferably 950° C. or lower. When the decompositiontemperature is 1200° C. or lower, it is possible to suppress unintendeddisproportionation of the silicon compound particle. After thetemperature inside the furnace is raised to a predetermined temperature,the carbon layer is formed on the silicon compound particle surface. Inthis manner, the negative electrode active material particle can beproduced. The hydrocarbon gas used as the raw material of the carbonmaterial is not particularly restricted; however, in the C_(n)H_(m)composition, n≤3 is preferable. When n≤3, the production cost thereofcan be made low, and also physical properties of the decompositionproducts can be made excellent.

Next, reformation is carried out by inserting Li into the negativeelectrode active material particle prepared as described above. In thisevent, Li₂Si₂O₅ is incorporated into the negative electrode activematerial particle. Li can be inserted, for example, by a thermal dopingmethod, an electrochemical method, or an oxidation-reduction method,preferably an oxidation-reduction method.

In the reformation by the oxidation-reduction method, for example,first, lithium can be inserted by immersing the silicon active materialparticle into a solution A in which lithium has been dissolved in anether solvent. This solution A may further contain a polycyclic aromaticcompound or a linear polyphenylene compound. After the lithiuminsertion, the silicon active material particle is immersed into asolution B containing a polycyclic aromatic compound or a derivativethereof, so that active lithium can be released from the silicon activematerial particle. As a solvent of the solution B, for example, anether-based solvent, a ketone-based solvent, an ester-based solvent, analcohol-based solvent, an amine-based solvent, or a mixed solventthereof can be used. Alternatively, after the immersion into thesolution A, the resulting silicon active material particle may be heatedin an inert gas. This heat treatment can stabilize the Li compound.Then, the resultant may be washed, for example, by a washing method withan alcohol, alkaline water in which lithium carbonate has beendissolved, a weak acid, pure water, or the like.

As the ether-based solvent used in the solution A, it is possible to usediethyl ether, tert-butylmethyl ether, tetrahydrofuran, dioxane,1,2-dimethoxy ethane, diethylene glycol dimethyl ether, triethyleneglycol dimethyl ether, tetraethylene glycol dimethyl ether, a mixedsolvent thereof, or the like. Among them, tetrahydrofuran, dioxane, or1,2-dimethoxy ethane is particularly preferably used. These solvents arepreferably dehydrated, and preferably deoxidized.

As the polycyclic aromatic compound contained in the solution A, it ispossible to use one or more of naphthalene, anthracene, phenanthrene,naphthacene, pentacene, pyrene, picene, triphenylene, coronene,chrysene, and derivatives thereof. As the linear polyphenylene compound,it is possible to use one or more of biphenyl, terphenyl, andderivatives thereof.

As the polycyclic aromatic compound contained in the solution B, it ispossible to use one or more of naphthalene, anthracene, phenanthrene,naphthacene, pentacene, pyrene, picene, triphenylene, coronene,chrysene, and derivatives thereof.

As the ether-based solvent of the solution B, it is possible to usediethyl ether, tort-butylmethyl ether, tetrahydrofuran, dioxane,1,2-dimethoxy ethane, diethylene glycol dimethyl ether, triethyleneglycol dimethyl ether, tetraethylene glycol dimethyl ether, and thelike.

As the ketone-based solvent, it is possible to use acetone,acetophenone, and the like.

As the ester-based solvent, it is possible to use methyl formate, methylacetate, ethyl acetate, propyl acetate, isopropyl acetate, and the like.

As the alcohol-based solvent, it is possible to use methanol, ethanol,propanol, isopropyl alcohol, and the like.

As the amine-based solvent, it is possible to use methylamine,ethylamine, ethylenediamine, and the like.

Next, the reformed negative electrode active material particle may bewashed.

Then, the washed negative electrode active material particle may beheated. More specifically, the heat treatment can be performed under anAr atmosphere at a heat treatment temperature in a range from 680° C. ormore to 770° C. or less. This temperature range is preferable becauseunintended disproportion of the silicon compound hardly progresses.Moreover, heating the reformed negative electrode active materialparticle can convert Li₄SiO₄ to Li₂SiO₃ and Li₂Si₂O₅. In the abovetemperature range, Li₄SiO₄ is converted mainly to Li₂Si₂O₅. In thereformation by the electrochemical method and the oxidation-reductionmethod, Li₄SiO₄ is likely to be formed as the Li silicate. Nevertheless,adjustment is also possible by this heat treatment in such a way as toconvert Li₄SiO₄ mainly to Li₂Si₂O₅, so that the peak intensity C becomesthe highest while the peak intensity A and the peak intensity C satisfythe relationship of the formula 1. In this case, the heat treatmenttemperature is more preferably higher than 680° C. or more but no higherthan 750° C. The heat treatment time in this event can be 1 hour or moreto 24 hours or less. Meanwhile, the Li silicate obtained by this methodhas some crystallinity but it cannot be said perfect.

Next, from the negative electrode active material particles, such anegative electrode active material particle is selected that the peakintensity C is the highest intensity and the peak intensity A and thepeak intensity C satisfy the relationship of the formula 1 among thepeak intensity A derived from Li₂SiO₃, the peak intensity B derived fromSi, the peak intensity C derived from Li₂Si₂O₅, and the peak intensity Dderived from SiO₂ which are obtained from the ²⁹Si-MAS-NMR spectrum ofthe negative electrode active material particle. The ²⁹Si-MAS-NMRspectrum may be measured under the above-described measurementconditions.

Note that the selection of the negative electrode active materialparticle does not have to be performed every time when the negativeelectrode active material is produced. Once the production conditionwith which the peak intensity C is the highest intensity and the peakintensity A and the peak intensity C satisfy the relationship of theformula 1 is found and selected, thereafter, the negative electrodeactive material can be produced with the same condition as the selectedcondition.

The negative electrode active material prepared in the way as mentionedabove is mixed with other materials such as the negative electrodebinder and the conductive assistant to obtain a negative electrodemixture. Then, an organic solvent, water, or the like is added theretoto obtain a slurry. Next, this slurry is applied onto the negativeelectrode current collector surface and then dried to form a negativeelectrode active material layer. At this time, a hot press or the likemay be carried out as needed. In the way as mentioned above, thenegative electrode can be produced.

<Lithium Ion Secondary Battery>

Next, a lithium ion secondary battery of the present invention will beexplained. The lithium ion secondary battery of the present inventionuses the negative electrode including the negative electrode activematerial of the present invention. Here, the laminate film type lithiumion secondary battery is taken as the specific example.

[Constitution of Laminate Film Type Lithium Ion Secondary Battery]

A laminate film type lithium ion secondary battery 20 shown in FIG. 2mainly has a rolled electrode body 21 stored inside an exterior member25 having a sheet form. This rolled body has a separator between apositive electrode and a negative electrode, and they are rolled.Alternatively, there is a case that a stacked body having the separatorbetween the positive electrode and the negative electrode is stored. Inboth of the electrode bodies, a positive electrode lead 22 is attachedto the positive electrode, and a negative electrode lead 23 is attachedto the negative electrode. The outermost periphery of the electrode bodyis protected by a protection tape.

The positive- and negative electrode leads are led out, for example, inone direction from inside to outside of the exterior member 25. Thepositive electrode lead 22 is formed of a conductive material such as,for example, aluminum; and the negative electrode lead 23 is formed of aconductive material such as, for example, nickel or copper.

The exterior member 25 is a laminate film in which, for example, amelt-adhering layer, a metal layer, and a surface protection layer arestacked in this order. In the laminate film, the melt-adhering layers oftwo films are melt-adhered or adhered by an adhesive or the like to eachother in the outer peripheral portions of the melt-adhering layers suchthat the melt-adhering layers may face to the electrode body 21. Forexample, the melt-adhering portion is a film of polyethylene,polypropylene, or the like, and the metal portion is an aluminum foil orthe like. The protection layer is, for example, nylon or the like.

Between the exterior member 25 and the positive- and negative electrodeleads, adhesion films 24 are inserted in order to prevent an outside airfrom invading. The material thereof is, for example, a polyethylene,polypropylene, or polyolefin resin.

[Positive Electrode]

The positive electrode has a positive electrode active material layer onboth surfaces or one surface of a positive electrode current collector,for example, in the same way as the negative electrode 10 of FIG. 1.

The positive electrode current collector is formed of a conductivematerial such as, for example, aluminum.

The positive electrode active material layer includes one, or two ormore of positive electrode materials which can adsorb and release alithium ion, and may include other materials such as a binder, aconductive assistant, and a dispersant, depending on the design thereof.In this case, details of the binder and the conductive assistant are,for example, the same as those of the negative electrode binder and thenegative electrode conductive assistant as described before.

The positive electrode material is preferably a lithium-containingcompound. Illustrative example of the lithium-containing compoundincludes a composite oxide formed of lithium and a transition metalelement, or a phosphoric acid compound having lithium and a transitionmetal element. Among these positive electrode materials, a compoundhaving at least one or more of nickel, iron, manganese, and cobalt ispreferable. These positive electrode materials are expressed by thechemical formula such as, for example, Li_(x)M1O₂ or Li_(y)M2PO₄. In theformulae, M1 and M2 represent at least one or more of transition metalelements. The values of x and y are different depending on the charge ordischarge state of the battery; but they are generally represented by0.05≤x≤1.10 and 0.05≤y≤1.10.

Illustrative example of the composite oxide having lithium and atransition metal element includes a lithium cobalt composite oxide(Li_(x)CoO₂), a lithium nickel composite oxide (Li_(x)NiO₂), and thelike. Illustrative example of the phosphoric acid compound havinglithium and a transition metal element includes a lithium iron phosphatecompound (LiFePO₄), a lithium iron manganese phosphate compound(LiFe_(1-u)Mn_(u)PO₄ (0<u<1)), and the like. When these positiveelectrode materials are used, not only a high battery capacity but alsoan excellent cycle characteristics can be obtained.

[Negative Electrode]

The negative electrode has the same constitution as that of the negativeelectrode 10 for the lithium ion secondary battery shown in FIG. 1,thereby having, for example, the negative electrode active materiallayers 12 on both surfaces of the current collector 11. In this negativeelectrode, the negative electrode charge capacity is preferably greaterthan the electric capacity (charge capacity as the battery) obtainedfrom the positive electrode active material. This is because depositionof the lithium metal on the negative electrode can be suppressed.

The positive electrode active material layers are arranged on part ofboth surfaces of the positive electrode current collector. The negativeelectrode active material layers are also arranged on part of bothsurfaces of the negative electrode current collector. In this case, forexample, the negative electrode active material layer formed on thenegative electrode current collector is provided with an area not facingto the positive electrode active material layer. This is for a stablebattery design.

In the non-facing area, that is, the area in which the negativeelectrode active material layer does not face to the positive electrodeactive material layer, there is hardly an influence of charge anddischarge. Because of this, the state of the negative electrode activematerial layer immediately after formation thereof can be maintained asit is. Accordingly, the composition, etc. of the negative electrodeactive material can be accurately and reproducibly investigatedirrespective of charge and discharge.

[Separator]

The separator separates the negative electrode and the positiveelectrode, and allows for the lithium ion to pass therethrough whilepreventing the short circuit due to contacts of both the electrodes.This separator is formed of, for example, a porous film made of asynthetic resin or ceramic, or may have a stacked structure of two ormore porous films. Illustrative example of the synthetic resin includespolytetrafluoroethylene, polypropylene, polyethylene, and the like.

[Electrolytic Solution]

At least a part of the active material layer or the separator isimpregnated with a liquid electrolyte (an electrolytic solution). Inthis electrolytic solution, an electrolyte salt is dissolved in asolvent, and other materials such as an additive may be also contained.

As the solvent, for example, a non-aqueous solvent may be used.Illustrative example of the non-aqueous solvent includes ethylenecarbonate, propylene carbonate, butylene carbonate, dimethyl carbonate,diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate,1,2-dimethoxy ethane, tetrahydrofuran, or the like. Among these, it isdesirable to use at least one of ethylene carbonate, propylenecarbonate, dimethyl carbonate, diethyl carbonate, and ethyl methylcarbonate. This is because better characteristics can be obtained. Inthis case, by using a high viscosity solvent such as ethylene carbonateor propylene carbonate in combination with a low viscosity solvent suchas dimethyl carbonate, ethyl methyl carbonate, or diethyl carbonate,more advantageous characteristics can be obtained. This is becausedissociation property and ion mobility of the electrolyte salt areimproved.

In the case of using an alloy-based negative electrode, it isparticularly desirable that at least one of a halogenated linearcarbonic acid ester and a halogenated cyclic carbon acid ester cccontained as a solvent. This is because a stable coating film is formedon the surface of the negative electrode active material at the time ofcharge and discharge, particularly at the time of charging. Thehalogenated linear carbonic acid ester is a carbonic acid ester having ahalogen as a constituent element (at least one hydrogen is substitutedby the halogen). The halogenated cyclic carbonic acid ester is a cycliccarbonic acid ester having a halogen as a constituent element (at leastone hydrogen is substituted by the halogen).

The kind of the halogen is not particularly restricted, and fluorine ismore preferable. This is because a coating film having better quality isformed than those by the other halogens. In addition, the number of thehalogens is desirably as large as possible. This is because the obtainedcoating film is more stable and the decomposition reaction of theelectrolytic solution is reduced.

Illustrative example of the halogenated linear carbonic acid esterincludes fluoromethyl methyl carbonate, difluoromethyl methyl carbonate,and the like. Illustrative example of the halogenated cyclic carbonicacid ester includes 4-fluoro-1,3-dioxolan-2-one,4,5-difluoro-1,3-dioxolan-2-one, and the like.

As a solvent additive, it is preferable to contain an unsaturated carbonbond cyclic carbonic acid ester. This is because a stable coating filmis formed on the surface of the negative electrode at the time of chargeand discharge, and the decomposition reaction of the electrolyticsolution can be suppressed. Illustrative example of the unsaturatedcarbon bond cyclic carbonic acid ester includes vinylene carbonate,vinyl ethylene carbonate, and the like.

In addition, it is preferable to include a sultana (cyclic sulfonateester) as the solvent additive. This is because a chemical stability ofthe battery is improved. Illustrative example of the sultone includespropane sultone and propene sultone.

Further, it is preferable that the solvent include an acid anhydride.This is because a chemical stability of the electrolytic solution isimproved. Illustrative example of the acid anhydride includes propanedisulfonic anhydride.

Illustrative example of the electrolyte salt can include any one or moreof a light metal salt such as a lithium salt. Illustrative example ofthe lithium salt includes lithium hexafluorophosphate (LiPF₆), lithiumterafluoroborate (LiBF₄), and the like.

The content of the electrolyte salt is preferably 0.5 mol/kg or more and2.5 mol/kg or less based on the content of the solvent. This is becausehigh ion conductivity can be obtained.

[Method for Producing Laminate Film Type Secondary Battery]

In the present invention, a negative electrode is produced by using thenegative electrode active material produced by the method for producingthe negative electrode active material of the present invention, and alithium ion secondary battery is produced using the produced negativeelectrode.

At first, a positive electrode is produced by using the positiveelectrode material as mentioned above. First, a positive electrodeactive material is mixed, as needed, with a binder, a conductiveassistant, etc. to obtain a positive electrode material mix, which isthen dispersed into an organic solvent to obtain a slurry of thepositive electrode mixture. Next, the mixture slurry is applied onto thepositive electrode current collector by using a coating apparatus suchas a die coater having a knife roll or a die head, and then dried by ahot air to obtain a positive electrode active material layer. Finally,the positive electrode active material layer is press molded with a rollpress machine or the like. In this event, heating may be performed, orheating or pressing may be repeated plural times.

Next, by following the same operation procedure as production of thenegative electrode 10 for the lithium ion secondary battery, thenegative electrode active material layer is formed on the negativeelectrode current collector to obtain the negative electrode.

Upon producing the positive electrode and the negative electrode, theactive material layer of each is formed on both surfaces of thepositive- and negative electrode current collectors. At this time, theactive material coating lengths on the both surface portions may differfrom each other in any of the electrodes (see FIG. 1).

Next, an electrolytic solution is prepared. Then, by ultrasonic weldingor the like, the positive electrode lead 22 is attached to the positiveelectrode current collector, and also the negative electrode lead 23 isattached to the negative electrode current collector as shown in FIG. 2.Subsequently, the positive electrode and the negative electrode arestacked or rolled with the separator therebetween to obtain the rolledelectrode body 21. The protection tape is adhered to the outermostperipheral portion of the rolled electrode body 21. Next, the rolledbody is molded into a flat form. Then, after the rolled electrode bodyis sandwiched by the folded exterior member 25 in a film form, theinsulation portions of the exterior member are adhered to each other bya thermal adhesion method so as to seal the rolled electrode body withan open state only in one direction. The adhesion films are insertedbetween the exterior member and the positive electrode lead and betweenthe exterior member and the negative electrode lead. A predeterminedamount of the electrolytic solution previously prepared is charged fromthe open portion for vacuum impregnation. After the impregnation, theopen portion is sealed by a vacuum thermal adhesion method. In the wayas mentioned above, the laminate film type lithium ion secondary battery20 can be produced.

EXAMPLES

Hereinafter, the present invention will be explained more specificallyby showing Examples and Comparative Examples of the present invention;but the present invention is not limited to these Examples.

Example 1-1

According to the following procedure, the laminate film type lithiumsecondary battery 20 shown in FIG. 2 was produced.

First, a positive electrode was prepared. A positive electrode mixturewas prepared by mixing 95% by mass of LiNi_(0.7)Co_(0.25)Al_(0.05)O,which is a lithium nickel cobalt aluminum composite oxide (NCA) andserves as a positive electrode active material, with 2.5% by mass of apositive electrode conductive assistant and 2.5% by mass of a positiveelectrode binder (polyvinylidene fluoride: PVDF). Next, the positiveelectrode mixture was dispersed into an organic solvent(N-methyl-2-pyrrolidone: NMP) to obtain a slurry in a paste form. Then,the slurry was applied onto both surfaces of the positive electrodecurrent collector by using a coating apparatus having a die head, anddried with hot-air drying equipment. At this time, the positiveelectrode current collector used had a thickness of 20 μm. Finally,press molding was carried out with a roll press.

Next, a negative electrode was prepared. First, a negative electrodeactive material was prepared as follows. A mixture of metal silicon andsilicon dioxide was introduced as the raw material into a reactionfurnace. The mixture was vaporized in an atmosphere with the vacuumdegree of 10 Pa and deposited onto an adsorbing plate. Aftersufficiently cooled, the deposited material was taken out and crushedwith a ball mill. The x value of SiO_(x) of the silicon compoundparticles thus obtained was 1.0. Then, the particle diameter of thesilicon compound particles was adjusted by classification. Subsequently,the thermal decomposition CVD was carried out to cover the siliconcompound particle surface with a carbon material. The carbon materialhad an average thickness of 100 nm.

Next, the silicon compound covered with the carbon material was reformedby inserting lithium into the silicon compound particles according tothe oxidation-reduction method. In this case, first, the negativeelectrode active material particles were immersed in a solution(solution C) in which lithium pieces and an aromatic compoundnaphthalene had been dissolved in tetrahydrofuran (hereinafter referredto as THF). The solution C was prepared by dissolving naphthalene into aTHF solvent at a concentration of 0.2 mol/L, and then adding lithiumpieces with a mass content of 10% by mass to the mixture solution of THFand naphthalene. Moreover, when the negative electrode active materialparticles were immersed, the temperature of the solution was 20° C., andthe immersion time was 20 hours. Thereafter, the negative electrodeactive material particles were collected by filtration. By theabove-described treatments, lithium was inserted into the negativeelectrode active material particles.

Next, the negative electrode active material particles were washed. Thewashed silicon compound particles were heated under an Ar atmosphere. Atthis time, the heat treatment was performed at 720 degrees. The heattreatment time was 3 hours. By the above-described treatments,crystalline Li₂Si₂O₅ was formed in the silicon compound particles.

Next, the ²⁹Si-MAS-NMR spectrum of the negative electrode activematerial particles thus prepared was measured. FIG. 4 shows the²⁹Si-MAS-NMR spectrum of Example 1-1. In the ²⁹Si-MAS-NMR spectrum, thebase line was set, and peak intensities were calculated. The peakintensity C derived from Li₂Si₂O₅ was the highest peak. Note that A isthe peak intensity appearing near −75 ppm; B, near −86 ppm; C, near −93ppm; and D, near −110 ppm. As a relationship of the peak intensities,A=C/10 (=0.1 C) which satisfied (formula 1) A<C/3 and (formula 2) A≤C/5.Moreover, the second highest peak intensity was B, and C was also theintensity five times or more as high as B. Further, the crystallinity ofthe Li silicates in the negative electrode active material particles wasconfirmed by an X-ray diffraction. A spectrum of Example 1-1 obtained bythe X-ray diffraction is shown in FIG. 5. In Example 1-1, the peakappearing near 2θ=18.5° is derived from Li₂SiO₃, and the peak appearingnear 2θ=24.6° is a peak derived from Li₂Si₂O₅. Note that FIG. 5 alsoshows a spectrum of Comparative Example 1-2 obtained by an X-raydiffraction, which will be described later.

Furthermore, from the negative electrode active material particles, thehalf-value width (2θ) of the diffraction peak attributable to the Si(111) crystal plane obtained by the X-ray diffraction using Cu-Kα raywas 1.86°, and the crystallite's size corresponding to the Si (111)crystal plane was 4.6 nm.

Furthermore, in the ²⁹Si-MAS-NMR measurement, a peak was also obtainedat a position in the vicinity of a chemical shift value of −130 ppm.Presumably, this peak is the peak derived from amorphous silicon.

The negative electrode active material particles had a median diameterof 4 μm.

Here, to evaluate the stability of the aqueous slurry containing thenegative electrode active material particles, 30 g of the preparednegative electrode mixture slurry was taken out as a portion separatefrom the slurry for preparing a secondary battery, and stored at 20° C.to measure the time after the negative electrode mixture slurrypreparation until gas generation.

The negative electrode active material particles (silicon-based negativeelectrode active material) thus prepared and a carbon-based activematerial were blended with the mass ratio of 1:9 to prepare a negativeelectrode active material. Here, natural graphite and artificialgraphite covered with a pitch layer were mixed with the mass ratio of5:5 for use as the carbon-based active material. The carbon-based activematerial had a median diameter of 20 μm.

Next, the prepared negative electrode active material, a conductiveassistant 1 (carbon nanotube, CNT), a conductive assistant 2 (carbonmicroparticles with a median diameter of about 50 nm), a styrenebutadiene rubber (styrene butadiene copolymer, hereinafter referred toas SBR), carboxymethyl cellulose (hereinafter referred to as CMC) weremixed with the dry mass ratio of 92.5:1:1:2.5:3, and diluted with purewater to obtain a negative electrode mixture slurry. Note that the aboveSBR and CMC are negative electrode binders.

As the negative electrode current collector, an electrolytic copper foilhaving a thickness of 15 μm was used. This electrolytic copper foilincluded carbon and sulfur each at a concentration of 70 ppm by mass.Finally, the negative electrode mixture slurry was applied onto thenegative electrode current collector and dried in a vacuum atmosphere at100° C. for 1 hour. After the drying, the deposited amount (alsoreferred to as area density) of the negative electrode active materiallayer per unit area on one surface of the negative electrode was 5mg/cm².

Next, after solvents (4-fluoro-1,3-dioxolan-2-one (FEC), ethylenecarbonate (EC), and dimethyl carbonate (DMC)) were mixed, an electrolytesalt (lithium hexafluorophosphate: LiPF₆) was dissolved therein toprepare an electrolytic solution. In this case, the solvent compositionwas FEC:EC:DMC=10:20:70 in terms of the volume ratio, and the content ofthe electrolyte salt in the solvents was 1.2 mol/kg.

Next, a secondary battery was fabricated as follows. First, an aluminumlead was welded to one end of the positive electrode current collectorby ultrasonic, and a nickel lead was welded to one end of the negativeelectrode current collector. Then, the positive electrode, a separator,the negative electrode, and a separator were stacked in this order androlled in the longitudinal direction to obtain a roiled electrode body.The roll end portion was fixed with a PET protection tape. As theseparators, a laminate film (thickness: 12 μm) was used in which a filmmainly containing porous polyethylene was sandwiched by films mainlycontaining porous polypropylene. Subsequently, after the electrode bodywas sandwiched by an exterior member, outer peripheral portions thereofwere thermally adhered to each other except for one side, so that theelectrode body was stored inside the exterior member. As the exteriormember, an aluminum laminate film was used in which a nylon film, analuminum foil, and a polypropylene film were stacked. Thereafter, fromthe open portion, the prepared electrolytic solution was charged forimpregnation under a vacuum atmosphere. Then, the open portion wassealed by thermal adhesion.

The cycle characteristics and the first time charge and dischargecharacteristics of the secondary battery thus prepared were evaluated.

The cycle characteristics were examined as follows. First, in order tostabilize the battery, two cycles of charge and discharge were carriedout at the atmospheric temperature of 25° C. with 0.2 C. The dischargecapacity at the second cycle was measured. Next, the charge anddischarge were carried out until the total cycle number reached 499cycles, and the discharge capacity was measured at each cycle. Finally,the discharge capacity at the 500th cycle obtained with the 0.2 C chargeand discharge was divided with the discharge capacity at the secondcycle to calculate the capacity retention rate (hereinafter, alsoreferred to as simply retention rate). In the normal cycle, that is,from the 3rd cycle to the 499th cycle, the charge and discharge wereperformed with the 0.7 C charge and the 0.5 C discharge.

In examining the first time charge and discharge characteristics, thefirst time efficiency (hereinafter, sometimes referred to as initialefficiency) was calculated. The first time efficiency was calculatedfrom the equation shown by: first time efficiency (%)=(first timedischarge capacity/first time charge capacity)×100. The atmosphere andtemperature were the same as those used in the cycle characteristicsexamination.

Further, from the negative electrode prepared as described above and acounter lithium, a coin battery type test cell with the size of 2032 wasfabricated; and the discharge behavior thereof was evaluated. Morespecifically, first, the constant current and constant voltage chargingwas carried out up to 0 V in the counter Li; and when the currentdensity reached 0.05 mA/cm², the charge was stopped. Thereafter, theconstant current discharge was carried out to 1.2 V. The current densityat this time was 0.2 mA/cm². This charge and discharge was repeated for30 times. From the data obtained in each charge and discharge, the graphwas plotted where the capacity change rate (dQ/dV) is shown in thevertical axis and the voltage (V) is shown in the horizontal axis so asto check whether or not a peak was obtained in the range of 0.4 to 0.55(V). As a result, within 30 times of the charge and discharge, the peakwas obtained in the range of 0.4 to 0.55 (V). Moreover, the peak wasobtained in every charge and discharge from the charge and discharge inwhich the peak was observed for the first time till the 30th charge anddischarge. Hereinafter, this peak is also referred to as dQ/dV peak.

Further, the first time efficiency of the silicon-based active materialalone (SiOx alone) was calculated as follows. First, the negativeelectrode active material particles (the silicon compound particlescovered with the carbon material) prepared above and polyacrylic acidwere mixed with the mass ratio of 85:15. This mixture was applied onto acopper foil. At this time, the area density of the applied mixture wasabout 2 mg/cm². Then, the resultant was vacuum dried at 90° C. for 1hour. Then, the constant current and constant voltage charging wasstarted with a voltage of 0 V and a current density of 0.2 mA/cm² usingthe counter Li in a coin battery with the size of 2032. Subsequently,when the current value reached 0.1 mA, the constant current and constantvoltage charging was stopped. Thereafter, the constant current dischargewas carried out, and the discharge was stopped when the voltage reached1.2 V. The current density during the discharging was the same as thatduring the charging. At this time, the first time efficiency of thesilicon-based active material alone (SiOx alone) is: (dischargecapacity)/(charge capacity)×100(%), given that the condition forinputting Li to the negative electrode represents the charge, and thecondition for taking out Li from the negative electrode represents thedischarge. Using this formula, the first time efficiency of SiOx alonewas calculated. As a result, the first time efficiency of SiOx alone wasabout 86%.

Comparative Example 1-1

In Comparative Example 1-1, the silicon compound particles covered withthe carbon material before the Li insertion in Example 1-1 were used asa silicon-based negative electrode active material, and mixed with thecarbon-based active material as in Example 1-1 to prepare a negativeelectrode active material. Next, a secondary battery was fabricated bythe same method as in Example 1-1, except that this negative electrodeactive material was used. Then, the cycle characteristics, first timeefficiency, and slurry stability were evaluated. FIG. 6 shows the²⁹Si-MAS-NMR spectrum of Comparative Example 1-1. As can be seen fromFIG. 6, the peak intensity D derived from SiO₂ is the highest. Inaddition, it was impossible to determine the relationship between A andC. Moreover, it was impossible to determine the peak near −130 ppm,either. Note that, in Comparative Example 1-1, the initial efficiency ofthe SiO material alone was calculated as in Example 1-1 and was about68%.

Comparative Example 1-2

In Comparative Example 1-2, a secondary battery was fabricated by thesame method as in Example 1-1, except that the heat treatment was notperformed after the Li insertion. Then, the cycle characteristics, firsttime efficiency, and slurry stability were evaluated. FIG. 7 shows the²⁹Si-MAS-NMR spectrum of Comparative Example 1-2. As can be seen fromFIG. 7, the peak intensity B derived from Si is the highest. Inaddition, it was impossible to determine the relationship between A andC. Moreover, a peak was obtained at a position in the vicinity of achemical shift value of −130 ppm. Further, the crystallinity of the Lisilicates in the negative electrode active material particles waschecked by an X-ray diffraction. The spectrum of Comparative Example 1-2obtained by the X-ray diffraction is shown in FIG. 5. In ComparativeExample 1-2, a peak derived from crystalline Li₂SiO₃ was observed at2θ=18.5°, but no peak derived from crystalline Li₂Si₂O₅ was observed at2θ=24.6°.

Comparative Example 1-3

A secondary battery was fabricated as in Example 1-1, except that theheat treatment temperature was changed to 500° C. to control the peakintensity A so as to be the highest peak intensity. Note that, inComparative Example 1-3, the relationship between the peak intensities Aand C was A=5.6 C which did not satisfy the formula 1.

Comparative Example 1-4

A secondary battery was fabricated as in Example 1-1, except that theheat treatment temperature was changed to control the peak intensities Aand C so as not to satisfy the relationship of the formula 1. Note that,in Comparative Example 1-4, the heat treatment temperature was 680° C.The peak intensity C is the highest peak, but the relationship betweenthe peak intensities A and C was A=C/3 which did not satisfy the formula1.

Table 1 shows the results of Example 1-1 and Comparative Examples 1-1 to1-4. In the subsequent tables, the symbol “∘” means the correspondingcriterion is satisfied, and the symbol “x” means the criterion is notsatisfied.

TABLE 1 SiOx: x = 1.0, D₅₀ = 4 μm, and 10% by mass, current collector:copper foil (including carbon and sulfur) positive electrode: NCA,carbon material: 100 nm. dQ/dV: present Peak Intensity Half- Siintensity Whether multiplier value (111) Cycle Initial Time C is formula1 of 2nd width crystallite retention efficie until gas highest ishighest 2θ size Peak at rate ncy generation peak satisfied peak (°) (nm)−130 ppm (%) (%) (hours) Comparative x impossible — 2.59 3.3 impossible75 84 stable Example 1-1 to to determine determine Comparative ximpossible — 2.59 3.3 present 83 89.2 6 Example 1-2 to determineComparative x x — 2.33 3.43 present 84 89.5 48 Example 1-3 (A = 5.6 C)Comparative ∘ x — 1.86 4.6 present 82 89.6 72 Example 1-4 (A = C/3)Example 1-1 ∘ ∘ ∘ 1.86 4.6 present 81 89.6 120 (A = C/10) (C = 58)

As can be seen from Table 1, in Example 1-1 in which the peak intensityC was the highest and C was greater than a value of the intensity Amultiplied by 3 as deduced from the formula 1, the time until the gasgeneration was 120 hours and the longest in comparison with ComparativeExamples 1-2 to 1-4 in which was inserted, and the slurry stability wasthe highest. This verified that the larger the amount of Li₂Si₂O₅formed, the more the slurry stability is improved. Meanwhile, inComparative Example 1-4 in which the peak intensity C was the highestbut the formula 1 was not satisfied, the time until the gas generationwas 72 hours and longer than those of other Comparative Examples, butthe slurry stability was consequently inferior to that of Example 1-1.Normally, a slurry for preparing a negative electrode requires a timeafter the preparation until the application onto a negative electrodecurrent collector in many cases; for example, when a slurry forpreparing a negative electrode is prepared on Friday, the slurry isoften applied onto a negative electrode current collector on Monday inthe next week. Hence, it is preferable to obtain such a slurry stabilitythat the time until the gas generation is longer then 72 hours.

On the other hand, since Li was not inserted in Comparative Example 1-1,the slurry stability was high, but the cycle characteristics and initialefficiency were low. Meanwhile, in Comparative Example 1-2, the initialefficiency and cycle characteristics were somewhat improved; however,since the heat treatment was not performed after the Li insertion, thecontent of Li₄SiO₄, which is easily dissolved in water, was high, andthe slurry stability was low and not suitable for industrial use.

In Comparative Example 1-3, since the peak intensity A was the highest,the formula 1 was not satisfied. In this case, the amount of Li₂SiO₃ waslarger than Li₂Si₂O₅. As a result, the cycle retention rate ofComparative Example 1-3 was improved in comparison with Example 1-1, butthe slurry stability was higher in Example 1-1 than in ComparativeExample 1-3.

Examples 2-1 to 2-3, Comparative Examples 2-1, 2-2

Secondary batteries were fabricated as in Example 1-1, except that theoxygen amount in the bulk of the silicon compound was adjusted. In thiscase, the oxygen amount was adjusted by changing the heating temperatureand the ratio of the metal silicon and the silicon dioxide it the rawmaterial of the silicon compound. Table 2 shows the values of x of thesilicon compound shown by SiO_(x) in Examples 2-1 to 2-3 and ComparativeExamples 2-1, 2-2. Note that, in Examples 2-1 to 2-3, C was the highestpeak intensity, and A and C satisfied the formula 1.

TABLE 2 SiOx: D₅₀ = 4 μm and 10% by mass, current collector: copper foil(including carbon and sulfur) positive electrode: NCA, carbon material:100 nm Time until Cycle Initial gas retention efficiency generation xdQ/dV rate (%) (%) (hours) Example 1-1 1 present 81 89.6 120 Comparative0.3 absent 65 91.3 48 Example 2-1 Example 2-1 0.5 absent 74 90.4 72Example 2-2 0.7 present 80.3 90.3 96 Example 2-3 1.5 present 80.5 89.6120 Comparative 1.8 present — — 120 Example 2-2

As shown in Table 2, when the value of x in the silicon compound shownby SiOx was within a range of 0.5≤x≤1.6, the battery characteristicswere further improved. When oxygen is not sufficient (x=0.3) as inComparative Example 2-1, the first time efficiency is improved, but thecapacity retention rate remarkably deteriorates. On the other hand, asrepresented by Comparative Example 2-2, when the oxygen amount is large(x=1.8), lithium is hardly adsorbed and released due to the excessiveamount of oxygen, and substantially the capacity of the silicon oxidewas not expressed. Hence, the evaluation was stopped.

Examples 3-1 to 3-4

Secondary batteries were fabricated under the same conditions as inExample 1-1, except that the crystallinity of the silicon compoundparticles was changed as shown in Table 3. Then, the cyclecharacteristics and first time efficiency were evaluated. Note that thecrystallinity in the silicon compound particles can be controlled bychanging the vaporization temperature of the raw material, or a heattreatment after the formation of the silicon compound particles.

TABLE 3 SiOx: x = 1, D₅₀ = 4 μm, and 10% by mass, current collector:copper foil (including carbon and sulfur) positive electrode: NCA,carbon material: 100 nm, dQ/dV peak: present, −130 ppm peak: presentHalf- Time until value Si (111) Cycle Initial gas width crystalliteretention efficiency generation 2θ (°) size (nm) rate (%) (%) (hours)Example 1-1 1.86 4.6 81 89.6 120 Example 3-1 1.77 4.91 80.5 89.7 120Example 3-2 1.22 7.19 80 89.5 120 Example 3-3 1.03 8.45 72 89.5 120Example 3-4 0.80 10.81 70 90 120

The capacity retention rate and first time efficiency were changeddepending on the crystallinity of the silicon compound particles.Especially, high capacity retention rates were obtained from lowcrystallinity materials in which the half-value width was 1.2° or moreand the crystallite's size attributable to the Si (111) plane was 7.5 nmor less.

Examples 4-1 to 4-5

Secondary batteries were fabricated under the same conditions as inExample 1-1, except that the median diameter of the negative electrodeactive material particles was changed as shown in Table 4. Then, thecycle characteristics and first time efficiency were evaluated.

TABLE 4 SiOx: x = 1 and 10% by mass, current collector: copper foil(including carbon and sulfur) C being the highest: satisfied, formula 1:satisfied, positive electrode: NCA, carbon material: 100 nm, dQ/dV peak:present, −130 ppm peak: present Time until Median Cycle Initial gasdiameter retention efficiency generation (μm) rate (%) (%) (hours)Example 1-1 4 81 89.6 120 Example 4-1 0.5 75 89.3 80 Example 4-2 1 7889.6 96 Example 4-3 10 82 90.0 120 Example 4-4 15 77 90.3 140 Example4-5 20 71 88.2 144

When the median diameter of the negative electrode active materialparticles was 1.0 μm or more, the cycle retention rate and the slurrystability were improved. This is presumably because the surface area ofthe negative electrode active material particle per a unit mass was nottoo large so that the area to cause a side reaction was successfullymade small. On the other hand, when the median diameter is 15 μm orless, the particle does not easily crack upon charging so that SEI(solid electrolyte interface) due to the new surface is difficult to beformed upon charging and discharging. Thus, the reversible Li loss canbe suppressed. In addition, when the median diameter of the negativeelectrode active material particles is 15 μm or less, expansion amountof the negative electrode active material particles upon charging is notso much that physical and electrical destruction of the negativeelectrode active material layer due to expansion can be prevented.

Examples 5-1 to 5-5

Secondary batteries were fabricated under the same conditions as inExample 1-1, except that the average thickness of the carbon materialcovering the surfaces of the silicon compound particles was changed.Then, the cycle characteristics and first time efficiency wereevaluated. The average thickness of the carbon material can be adjustedby changing the CVD conditions.

TABLE 5 SiOx: x = 1 and 10% by mass, current collector: copper foil(including carbon and sulfur) C being the highest: satisfied, formula 1:satisfied, positive electrode: NCA, dQ/dV peak: present, −130 ppm peak:present Carbon material Time until average Cycle Initial gas thicknessretention efficiency generation (nm) rate (%) (%) (hours) Example 1-1100 81 89.6 120 Example 5-1 0 76 88.5 84 Example 5-2 5 80 89.3 96Example 5-3 500 82 89.4 120 Example 5-4 1000 82 89.5 120 Example 5-55000 83 88.8 120

As can be seen from Table 5, when the average thickness of the carbonmaterial is 5 nm or more, the conductivity is especially improved. Thus,the capacity retention rate and initial efficiency can be improved. Onthe other hand, when the average thickness of the carbon material is5000 nm or less, a sufficient amount of the silicon compound particlescan be ensured in the battery design, so that the battery capacity isnot decreased.

Example 6-1

A secondary battery was fabricated under the same conditions as inExample 1-1, except that an electrolytic copper foil including no carbonand sulfur elements was used as the negative electrode currentcollector. Then, the cycle characteristics and first time efficiencywere evaluated.

TABLE 6 SiOx: x = 1 and 10% by mass, C being the highest: satisfied,formula 1: satisfied, positive electrode: NCA, carbon material: 100 nm,dQ/dV peak: present, −130 ppm peak: present Presence or absence ofcarbon Time until and sulfur Cycle Initial gas elements in retentionefficiency generation copper foil rate (%) (%) (hours) Example 1-1present 81 89.6 120 Example 6-1 absent 79 89.5 120

When the negative electrode current collector includes carbon and sulfureach in an amount of 70 ppm by mass or less, the strength of the currentcollector is improved. Thus, in a case of using a silicon-based negativeelectrode active material which largely expands and contracts duringcharge and discharge of the secondary battery, it is possible tosuppress the deformation and distortion of the current collector due tosuch expansion and contraction, and the cycle characteristics arefurther improved as in Example 1-1.

Example 7-1

Secondary batteries were fabricated under the same conditions as inExample 1-1, except that the ratio of the mass of the silicon-basedactive material particles in the negative electrode active material waschanged. Then, the percent increase in the battery capacity wasevaluated.

Comparative Example 7-1

Secondary batteries were fabricated under the same conditions as inComparative Example 1-1, except that the ratio of the mass of thesilicon-based active material particles in the negative electrode activematerial was changed. Then, the percent increase in the battery capacitywas evaluated. Specifically, the silicon active material particles usedhere were not subjected to any of the Li insertion and the subsequentheat treatment. The percent increase in the battery capacity wascalculated on the basis that the ratio of the silicon-based activematerial particles was 0% by mass (i.e., the carbon active material was100% by mass).

Table 7 shows as relationship between the ratio of the silicon-basedactive material particles relative to the total amount of the negativeelectrode active material and the percent increase in the batterycapacity of the secondary battery.

TABLE 7 Ratio of silicon- Percent increase in based active batterycapacity (%) material particles Comparative (% by mass) Example 7-1Example 7-1 0 0 0 10 8.2 7.1 20 17 11.3 30 22.8 13.9

As can be seen from Table 7, when the ratio of the silicon compound was10% by mass or more, the difference in the percent increase in thebattery capacity was widened between Example 7-1 and Comparative Example7-1. Thus, it was verified that when the ratio of the silicon compoundin the battery of the present invention is 10% by mass or more, theenergy density per volume of the battery is especially remarkablyincreased.

It should be noted that the present invention is not restricted to theabove-described embodiments. The embodiments are merely examples so thatany embodiments that have substantially the same feature and demonstratethe same functions and effects as those in the technical concept asdisclosed in claims of the present invention are included in thetechnical scope of the present invention.

The invention claimed is:
 1. A negative electrode active material comprising a negative electrode active material particle, wherein the negative electrode active material particle comprises a silicon compound particle comprising a silicon compound (SiO_(x): 0.5≤x≤1.6), the silicon compound particle comprises crystalline Li₂Si₂O₅ in at least part of the silicon compound particle, among a peak intensity A derived from Li₂SiO₃, a peak intensity B derived from Si, a peak intensity C derived from Li₂Si₂O₅, and a peak intensity D derived from SiO₂ which are obtained from a ²⁹Si-MAS-NMR spectrum of the silicon compound particle, the peak intensity C is the highest intensity, the peak intensity A and the peak intensity C satisfy a relationship of the following formula 1, A<C/3, and  formula 1 in the negative electrode active material, a half-value width (2θ) of a diffraction peak attributable to a Si (111) crystal plane obtained by an X-ray diffraction using Cu-Kα ray is 1.2° or more, and a crystallite's size corresponding to the crystal plane is 7.5 nm or less.
 2. The negative electrode active material according to claim 1, wherein the peak intensity A and the peak intensity C in the ²⁹Si-MAS-NMR spectrum further satisfy a relationship of the following formula 2, A≤C/5.  formula 2
 3. The negative electrode active material according to claim 1, wherein a peak appears in a region near a chemical shift value of −130 ppm in the ²⁹Si-MAS-NMR spectrum.
 4. The negative electrode active material according to claim 1, wherein the negative electrode active material particles have a median diameter of 1.0 μm or more and 15 μm or less.
 5. The negative electrode active material according to claim 1, wherein the negative electrode active material particle contains a carbon material at a surface layer portion.
 6. The negative electrode active material according to claim 5, wherein the carbon material to be coated has an average thickness of 5 nm or more and 5000 nm or less.
 7. A mixed negative electrode active material comprising: the negative electrode active material according to claim 1; and a carbon-based active material.
 8. The mixed negative electrode active material according to claim 7, wherein a ratio of the negative electrode active material relative to a total amount of the mixed negative electrode active material is 10% by mass or more.
 9. A method for producing a negative electrode active material which comprises a negative electrode active material particle comprising a silicon compound particle, the method comprising the steps of: making silicon compound particles comprising a silicon compound (SiO_(x): 0.5≤x≤1.6); inserting lithium into the silicon compound particles so as to include crystalline Li₂Si₂O₅ in at least part of the silicon compound particles to prepare negative electrode active material particles; selecting, from the negative electrode active material particles, such a negative electrode active material particle that, among a peak intensity A derived from Li₂SiO₃, a peak intensity B derived from Si, a peak intensity C derived from Li₂Si₂O₅, and a peak intensity D derived from SiO₂ which are obtained from a ²⁹Si-MAS-NMR spectrum of the negative electrode active material particle, the peak intensity C is the highest intensity, and the peak intensity A and the peak intensity C satisfy a relationship of the following formula 1 and in the negative electrode active material, a half-value width (2θ) of a diffraction peak attributable to a Si (111) crystal plane obtained by an X-ray diffraction using Cu-Kα ray is 1.2° or more, and a crystallite's size corresponding to the crystal plane is 7.5 nm or less; and producing a negative electrode active material by using the selected negative electrode active material particle, A<C/3.  formula 1 